Eccentric rotating mass (ERM) motors provide tactile feedback to a user of a device using an ERM motor by providing a shake or vibration that the user can feel. ERM motors may be used in cellular telephones to produce a vibration instead of an audible signal that notifies the telephone user of an event, such as an incoming call. An ERM motor spins a weight that is offset from an axis of rotation, which causes the ERM motor to shake or vibrate.
The ERM motors are driven by ERM drivers. In order to control the velocity or angular frequency of the motors, the drivers may operate in a closed loop mode. The closed loop mode uses a back electromechanical force (BEMF) voltage as the feedback signal. The BEMF contains a voltage proportional to the angular frequency of the motor and a ripple that can be significant depending on the motor construction. The ripple in the feedback signal propagates to the ERM driver causing undesired driving fluctuations. A filter can be inserted in the feedback path to attenuate the ripple; however, the filter introduces new poles in the gain of the feedback path, which degrade the stability of the motor.